The present invention relates generally to switch mode AC/DC power supplies. More particularly, the present invention pertains to switch mode AC/DC power converters that employ a front-end power factor correction circuit and active rectification devices to improve the efficiency of the converter.
Designers of AC/DC power supplies are constantly striving to improve the efficiency of their designs. At the same time, careful attention must be paid to the cost, component count, and physical size of the design.
Conventional AC/DC switch mode power supplies typically include multiple stages, modules, or sections of circuit elements that perform different functions in a process of efficiently converting power from an AC supply to a DC output. For example, a critical functional block in high efficiency AC/DC power conversion is a power factor correction (PFC) circuit to improve the quality of power drawn from the power grid to which a powered device is connected.
FIG. 1 shows the functional blocks arranged in a typical prior art AC/DC switch mode power supply 10. First, an input filter stage 11 filters unwanted high frequency noise originating from the high frequency switching within the AC power supply and protects the power supply from transients which may occur on the AC power source. An input passive rectification stage 12 uses a diode bridge rectifier to rectify the filtered AC input voltage. A power factor correction (PFC) stage 13 may include an active circuit that shapes the input current from the AC power source to a near sinusoidal shape. A protection device 14 bridges the PFC stage to protect the power supply from input voltage surges. A bulk capacitor 15 acts as a waveform smoothing and energy storage element to provide a near DC voltage to subsequent conversion stages. A DC/AC converter block 16 is conventionally a controlled switching circuit that converts the bulk DC voltage to a high frequency AC signal. The AC signal is coupled to the primary side of a transformer 17 that transforms the high frequency AC current/voltage as needed and provides galvanic isolation. The secondary side of the transformer 17 is then coupled to an output passive rectifier stage 18 that rectifies the high frequency AC signal to a DC voltage. Finally, an output filter stage 19 filters any residual AC components to provide a clean DC output to a DC load terminal.
Conventional AC/DC power supplies using multiple conversion stages as shown in FIG. 1 have relatively poor efficiency. For example, passive rectification of the AC input generates losses because of voltage drops in the multiple passive rectification components, e.g., diodes. These same losses occur in the output passive rectification stage.
Many AC/DC switch mode power supplies employ MOSFET switches in an active rectification circuit. MOSFET devices have an intrinsic (body drain) diode that is known to generate high reverse recovery losses when the diode changes from a conductive to a non-conductive state. Preferably, a high efficiency AC/DC power converter will minimize or avoid such losses.
What is needed is a switch mode power supply that is more energy efficient by eliminating circuit components and discrete conversion states that create unwanted losses.